1. Field of the Invention
This invention relates to polyvinyl halide compositions that possess dimensional stability to heat, impact strength and clarity. More particularly, the invention relates to a polyvinyl halide composition which is excellent in optical properties, impact resistance and heat resistance and can be employed in a wide variety of applications such as films, sheets, pipes, cable ducts, deck plates, building materials, battery housings, and industrial components.
2. Description of the Related Art
Polyvinyl chloride resins (PVC) are well known thermoplastic resins that can be compounded with a variety of property enhancing additives and molded, extruded, calendered or formed for a variety of applications. The art of PVC compounding is extremely complex in that many ingredients interact with each other. This complexity means that levels of ingredients and types of ingredients must be varied to yield a PVC compound that is useful for a given application.
Articles such as bottles, packages and films for certain markets demand a high degree of clarity. A property of PVC that is important for many applications is its inherent clarity and transparency. However, PVC resins are generally quite brittle. To overcome this deficiency, rubbery impact modifiers having glass transition temperatures below that of PVC are incorporated into the resin. Impact modifiers based on graft copolymers prepared by conventional radical polymerization are well known and have been widely employed for over thirty years. Graft copolymers of styrene, α-methylstyrene, acrylonitrile, methacrylic acid esters or mixtures thereof on a rubber, e.g., polybutadiene or a butadiene copolymer, and mixtures of such graft polymers with polystyrene or styrene copolymers are well known PVC impact modifiers. A variety of clear PVC impact modifiers are now commercially available. These impact modifiers possess refractive indices that closely match that of PVC in order to preserve the clarity and transparency of the PVC resin.
The freezing temperature (glass transition temperature) of PVC is about 80° C. so that its dimensional stability to heat (Vicat temperatures of about 75° to 84° C.) is insufficient for many applications where prolonged exposure to heat is encountered. Attempts have been made to improve the dimensional stability (heat resistance) of PVC by modifying the molecular structure of PVC, carrying out chemical after-treatments or adding thermoplastic resins with higher glass transition temperatures to the PVC base resin. Typical heat distortion temperature (HDT) modifiers can be used to raise HDT, but the result is an opaque compound that does not allow for use in PVC compounds where clarity is required. Known HDT modifiers that are opaque include acrylonitrile-α methylstyrene copolymer, acrylonitrile-α methylstyrene-butadiene copolymer and polymethyl methacrylate-acrylic ester copolymer (PMMA).
U.S. Pat. No. 5,166,271 discloses a heat resistant PVC copolymer obtained by graft copolymerizing a vinyl chloride resin with N-substituted maleimide in the presence of a radical polymerizable monomer which (1) is liquid at the temperature of copolymerization, (2) is capable of dissolving the N-substituted maleimide and (3) has a glass transition temperature of the polymer of 70° C. or more.
U.S. Pat. No. 5,206,296 discloses a processing aid for polymeric compositions such as PVC comprising a high molecular weight terpolymer comprising 10-49% by weight of a vinyl aromatic monomer such as styrene, 5-35% by weight of acrylonitrile, and 21-60% of an alkyl acrylate or alkyl methacrylate. The terpolymer disclosed in U.S. Pat. No. 5,206,296 possesses a viscosity of more than four as measured in dimethylformamide as 30° C. at a concentration of 100 mg terpolymer per 100 ml dimethylformamide. The average molecular weights of the terpolymer processing aid exceed 2,000,000, and are used at levels of 0.1 to 20 parts by weight per 100 parts by weight of PVC.
U.S. Pat. No. 5,354,812 discloses ternary alloys of PVC, post-chlorinated PVC (CPVC), and alloying polymers wherein the alloying polymers enhance the heat distortion temperature of the CPVC.
A combination of clear impact modifier and clear heat modifier that provides improved PVC compositions characterized by their clarity (light transmission), heat distortion temperature, and impact strength would be a useful advance in the PVC compounding art.